DIYbanter - View Single Post

On 25 Mar 2006 15:18:08 -0800, wrote:

Mikester wrote:
Cool idea for the fridge pump, never thought of that. I was thinking
of butchering an old portable air compressor.

Fridge Pump can work, but they are lubricated by oil that is supposed
to be circulating in the freon. Eventually, they will act up and sieze.
When they run, they will often SPIT oil out (and often it stinks), and
you can certainly feed some of it or new oil back in to add to
longevity.

I suggest you search ebay and buy a Gast or similar "rotary vane" pump.
This is NOT a piston pump, but carbon vanes that run in a smooth bore
cylinder, not much unlike a Wankle. No oil... continuous duty.... You
can put a checkvalve in the vac line, hang on a tank and cycle them
like a compressor.

I do not have the math in front of me, but you would have enough
surface area on both of the parts you speak of with 25-28" of vacuum
(easily provided by a Gast pump as mentioned), provided you do not get
overly wide with gasket material on that 2.5" puck. Then again, its
plastic, and you likely will not exert a lot of cutting pressure with
sharp tooling.

The 4x4 plate, depending on thickness could actually bow or bend with
even far less vacuum than I mentioned. So, you may need to provide
support in whatever jig you make.

When it comes to vacuum jigs, the DEPTH of the vacuum area is NOT
important. It is all about the SURFACE AREA you can provide the vacuum
access to. The bigger the surface area, the more you need to be
concerned about the vacuum pulling the part into a bow, IF you only
have a perimeter seal.

Vacuum is no weany way of holding stuff..... You can pull a hinder out
of a skunk with 29" of vacuum !

The next rule of thumb is you only need the amount of CFM that you are
LOSING via your gasketing. In other words, if you get a great seal, you
could suck it down once and it would effectively stay down until you
released the vacuum. So, this means you do NOT necessarily need a
monster pump. I only have a 1/3 horse unit, and as long as I have a
good seal, a 50 horse unit would NOT hold it any better.

For a seal, Use Soft Sheet rubber, Closed Cell foam (Volara, the stuff
under padded vinyl tops), or even closed cell weatherstripping tape if
necessary.

Vacuum jigs can be made from anything not pourous. Plastic, aluminum,
phenolics, corian, etc. If you know someone who can get you some short
lengths of various larger diameter acrylic tubing, these when flown off
parallel, Vac port in the side and a gasket top and bottom make a quick
vac jig.

Grummy
Greetings,
What Grummy says. Also, it's best to have the seal compress completely
so that the part does not warp. When making the vacuum chuck (the
plate you will have the vacuum passing through and which your part
sits on) think about how the vacuum will be applied to the part. If
you start with a very flat part, and a very flat chuck, when the part
is drawn tight against the chuck the vacuum will only be present at
the holes. I have found that cutting narrow grooves across the chuck
helps to keep the maximum surface areaexposed to vacuum.
Cheers,
Eric